Muted change in Atlantic overturning circulation over some glacial-aged Heinrich events

Abstract

Heinrich events - surges of icebergs into the North Atlantic Ocean - punctuated the last glacial period. The events are associated with millennial-scale cooling in the Northern Hemisphere. Freshwater from the melting icebergs is thought to have interrupted the Atlantic meridional overturning circulation, thus minimizing heat transport into the northern North Atlantic. The northward flow of warm water passes through the Florida Straits and is reflected in the distribution of seawater properties in this region. Here we investigate the northward flow through this region over the past 40,000 years using oxygen isotope measurements of benthic foraminifera from two cores on either side of the Florida Straits, which allow us to estimate water density, which is related to flow via the thermal wind relation. We infer a substantial reduction of flow during Heinrich Event 1 and the Heinrich-like Younger Dryas cooling, but little change during Heinrich Events 2 and 3, which occurred during an especially cold phase of the last glacial period. We speculate that because glacial circulation was already weakened before the onset of Heinrich Events 2 and 3, freshwater forcing had little additional effect. However, low-latitude climate perturbations were observed during all events. We therefore suggest these perturbations may not have been directly caused by changes in heat transport associated with Atlantic overturning circulation as commonly assumed.